Engineering Fundamentals of the
Internal Combustion Engine
.
iWillard W. Pulkrabek
University of Wisconsin-· .. Platteville
vi Contents
2-3 Mean Effective Pressure, 49 2-4 Torque and Power, 50 2-5 Dynamometers, 53
2-6 Air-Fuel Ratio and Fuel-Air Ratio, 55 2-7 Specific Fuel Consumption, 56
2-8 Engine Efficiencies, 59
2-9 Volumetric Efficiency, 60 ,
2-10 Emissions, 62 2-11 Noise Abatement, 62
2-12 Conclusions-Working Equations, 63 Problems, 65
Design Problems, 67
3 ENGINE CYCLES 68
3-1 Air-Standard Cycles, 68 3-2 Otto Cycle, 72
3-3 Real Air-Fuel Engine Cycles, 81 3-4 SI Engine Cycle at Part Throttle, 83 3-5 Exhaust Process, 86
3-6 Diesel Cycle, 91 3-7 Dual Cycle, 94
3-8 Comparison of Otto, Diesel, and Dual Cycles, 97 3-9 Miller Cycle, 103
3-10 Comparison of Miller Cycle and Otto Cycle, 108 3-11 Two-Stroke Cycles, 109
3-12 Stirling Cycle, 111 3-13 Lenoir Cycle, 113 3-14 Summary, 115
Problems, 116 Design Problems, 120
4 THERMOCHEMISTRY AND FUELS 121
4-1 Thermochemistry, 121
4-2 Hydrocarbon Fuels-Gasoline, 131
4-3 Some Common Hydrocarbon Components, 134 4-4 Self-Ignition and Octane Number, 139
4-5 Diesel Fuel, 148 4-6 Alternate Fuels, 150 4-7 Conclusions, 162
Problems, 162 Design Problems, 165
Contents
vii
5 AIR AND FUEL INDUCTION
166 5-1 Intake Manifold, 166
5-2 Volumetric Efficiency of SI Engines, 168 5-3 Intake Valves, 173
5-4 Fuel Injectors, 178 5-5 Carburetors, 181
5-6 Supercharging and Turbocharging, 190 5-7 Stratified Charge Engines
and Dual Fuel Engines, 195
5-8 Intake for Two-Stroke Cycle Engines, 196 5-9 Intake for CI Engines, 199
5-10 Conclusions, 201 Problems, 202 Design Problems, 204
6 FLUID MOTION WITHIN COMBUSTION CHAMBER
206 6-1 Turbulence, 206
6-2 Swirl, 208
6-3 Squish and Tumble, 213
6-4 Divided Combustion Chambers, 214 6-5 Crevice Flow and Blowby, 215 6-6 Mathematical Models and Computer
Simulation, 219
6-7 Internal Combustion Engine Simulation Program, 221
6-8 Conclusions, 225 Problems, 226 Design Problems, 228 7 COMBUSTION
229 7-1 Combustion in SI Engines, 229
7-2 Combustion in Divided Chamber Engines and Stratified Charge Engines, 243 7-3 Engine o?Itrating Characteristics, 246
7-4 Modern Fast Burn Combustion Chambers, 248 7-5 Combustion in CI Engines, 251
7-6 Summary, 259 Problems, 260 Design Problems, 261
viii Contents
8 EXHAUST FLOW 262
8-1 Blowdown, 262 8-2 Exhaust Stroke, 265 8-3 Exhaust Valves, 268 8-4 Exhaust Temperature, 269 8-5 Exhaust Manifold, 270 8-6 Turbochargers, 272
8-7 Exhaust Gas Recycle-EGR, 273 8-8 Tailpipe and Muffler, 273
8-9 Two-Stroke Cycle Engines, 274 8-10 Summary and Conclusions, 274
Problems, 275 Design Problems, 276
9 EMISSIONS AND AIR POLLUTION 277
9-1 Air Pollution, 277 9-2 Hydrocarbons (He), 278 9-3 Carbon Monoxide (CO), 285 9-4 Oxides of Nitrogen (NOx), 285 9-5 Particulates, 287
9-6 Other Emissions, 290 9-7 Aftertreatment, 292 9-8 Catalytic Converters, 293 9-9 CI Engines, 301
9-10 Chemical Methods to Reduce Emissions, 303 9-11 Exhaust Gas Recycle-EGR, 304
9-12 Non-Exhaust Emissions, 307 Problems, 308
Design Problems, 311
10 HEAT TRANSFER IN ENGINES 312
10-1 Energy Distribution, 313 10-2 Engine Temperatures, 314
10-3 Heat Transfer in Intake System, 317
10-4 Heat Transfer in Combustion Chambers, 318 10-5 Heat Transfer in Exhaust System, 324 10-6 Effect of Engine Operating Variables
on Heat Transfer, 327 10-7 Air Cooled Engines, 334 10-8 Liquid Cooled Engines, 335
~~~ ~ 10-9 Oil as a Coolant, 340
10-10 Adiabatic Engines, 341
10-11 Some Modern Trends in Engine Cooling, 342 10-12 Thermal Storage, 343
10-13 Summary, 345 Problems, 345 Design Problems, 348
11 FRICTION AND LUBRICATION 349
11-1 Mechanical Friction and Lubrication, 349 11-2 Engine Friction, 351
11-3 Forces on Piston, 360
11-4 Engine Lubrication Systems, 364 11-5 Two-Stroke Cycle Engines, 366 11-6 Lubricating Oil, 367
11-7 Oil Filters, 373
11-8 Summary and Conclusions, 375 Problems, 376
Design Problems, 377
APPENDIX 378
A-I Thermodynamic Properties of Air, 379 A-2 Properties of Fuels, 380
A-3 Chemical Equilibrium Constants, 381
A-4 Conversion Factors for Engine Parameters, 382
REFERENCES 384
ANSWERS TO SELECTEDREVIEW PROBLEMS 392
INDEX 395
This book was written to be used as an applied thermoscience textbook in a one- semester, college-level, undergraduate engineering course on internal combustion engines. It provides the material needed for a basic understanding of the operation of internal combustion engines. Students are assumed to have knowledge of funda- mental thermodynamics, heat transfer, and fluid mechanics as a prerequisite to get maximum benefit from the text. This book can also be used for self-study and/or as a reference book in the field of engines.
Contents include the fundamentals of most types of internal combustion engines, with a major emphasis on reciprocating engines. Both spark ignition and compression ignition engines are covered, as are those operating on four-stroke and two-stroke cycles, and ranging in size from small model airplane engines to the largest stationary engines. Rocket engines and jet engines are not included. Because of the large number of engines that are used in automobiles and other vehicles, a major emphasis is placed on these.
The book is divided into eleven chapters. Chapters 1 and 2 give an introduc- tion, terminology, definitions, and basic operating characteristics. This is followed in Chapter 3 with a detailed analysis of basic engine cycles. Chapter 4 reviews fun- damental thermochemistry as applied to engine operation and engine fuels.
Chapters 5 through 9 follow the air-fuel charge as it passes sequentially through an engine, including intake, motion within a cylinder, combustion, exhaust, and emis-
xi
xii Preface
sions. Engine heat transfer, friction, and lubrication are covered in Chapters 10 and 11. Each chapter includes solved example problems and historical notes followed by a set of unsolved review problems. Also included at the end of each chapter are open-ended problems that require limited design application. This is in keeping with the modern engineering education trend of emphasizing design. These design prob- lems can be used as a minor weekly exercise or as a major group project. Included in the Appendix is a table of solutions to selected review problems.
Fueled by intensive commercial competition and stricter government regula- tions on emissions and safety, the field of engine technology is forever changing. It is difficult to stay knowledgeable of all advancements in engine design, materials, con- trols, and fuel development that are experienced at an ever-increasing rate. As the outline for this text evolved over the past few years, continuous changes were required as new developments occurred. Those advancements, which are covered in this book, include Miller cycle, lean burn engines, two-stroke cycle automobile engines, variable valve timing, and thermal storage. Advancements and technologi- cal changes will continue to occur, and periodic updating of this text will be required.
Information in this book represents an accumulation of general material col- lected by the author over a period of years while teaching courses and working in research and development in the field of internal combustion engines at the Mechanical Engineering Department of the University of Wisconsin-Platteville.
During this time, information has been collected from many sources: conferences, newspapers, personal communication, books, technical periodicals, research, prod- uct literature, television, etc. This information became the basis for the outline and notes used in the teaching of a class about internal combustion engines. These class notes, in turn, have evolved into the general outline for this textbook. A list of ref- erences from the technical literature from which specific information for this book was taken is included in the Appendix in the back of the book. This list will be referred to at various points throughout the text. A reference number in brackets will refer to that numbered reference in the Appendix list.
Several references were of special importance in the development of these notes and are suggested for additional reading and more in-depth study. For keeping up with information about the latest research and development in automobile and internal combustion engine technology at about the right technical level, publica- tions by SAE (Society of Automotive Engineers) are highly recommended;
Reference [11] is particularly appropriate for this. For general information about most engine subjects, [40,58,100,116] are recommended. On certain subjects, some of these go into much greater depth than what is manageable in a one-semester course. Some of the information is slightly out of date but, overall, these are very informative references. For historical information about engines and automobiles in general, [29, 45, 97, 102] are suggested. General data, formulas, and principles of engineering thermodynamics and heat transfer are used at various places through- out this text. Most undergraduate textbooks on these subjects would supply the needed information. References [63] and [90] were used by the author.
Preface xiii Keeping with the trend of the world, SI units are used throughout the book, often supplemented with English units. Most research and development of engines is done using SI units, and this is found in the technical literature. However, in the non-technical consumer market, English units are still common, especially with automobiles. Horsepower, miles per gallon, and cubic inch displacement are some of the English terminology still used. Some example problems and some review prob- lems are done with English units. A conversion table of SI and English units of common parameters used in engine work is induded in the Appendix at the back of the book.
I would like to express my gratitude to the many people who have influenced me and helped in the writing of this book. First I thank Dorothy with love for always being there, along with John, Tim, and Becky. I thank my Mechanical Engineering Department colleagues Ross Fiedler and Jerry Lolwing for their assistance on many occasions. I thank engineering students Pat Horihan and Jason Marcott for many of the computer drawings that appear in the book. I thank the people who reviewed the original book manuscript and offered helpful suggestions for additions and improvements. Although I have never met them, I am indebted to authors J. B.
Heywood, C. R. Ferguson, E. F. Obert, and R. Stone. The books these men have written about internal combustion engines have certainly influenced the content of this textbook. I thank my father, who many years ago introduced me to the field of automobiles and generated a lifelong interest. I thank Earl of Capital City Auto Electric for carrying on the tradition.
ACKNOWLEDGMENTS
The author wishes to thank and acknowledge the following organizations for per- mission to reproduce photographs, drawings, and tables from their publications in this text: Carnot Press, Fairbanks Morse Engine Division of Coltec Industries, Ford Motor Company, General Motors, Harley Davidson, Prentice-Hall Inc., SAE Inter- national, Th~. Combustion Institute, and Tuescher Photography.
xvi Notation
CDt Discharge coefficient of carburetor throat CI Cetane index
CN Cetane number
EGR Exhaust gas recycle [%]
F Force [N] [lbf]
Ff Friction force [N] [lbf]
Fr Force of connecting rod [N] [lbf]
Fx Forces in the X direction [N] [lbf]
Fy Forces in the Y direction [N] [lbf]
Fl-2 View factor
FA Fuel-air ratio [kgf/kga] [lbmf/lbma]
FS Fuel sensitivity
I Moment of inertia [kg-m2 ] [lbm-ft2 ] ID Ignition delay [sec]
Ke Chemical equilibrium constant
M Molecular weight (molar mass) [kg/kgmole] [lbm/lbmmole]
MON Motor octane number
N Engine speed [RPM]
N Number of moles Nc Number of cylinders Nv Moles of vapor Nu Nusselt number ON Octane number
P Pressure [kPa] [atm] [psi]
Pa Air pressure [kPa] [atm] [psi]
Pex Exhaust pressure [kPa] [atm] [psi]
PEVO Pressure when the exhaust valve opens [kPa] [psi]
Pf Fuel pressure [kPa] [atm] [psi]
Pi Intake pressure [kPa] [atm] [psi]
Pinj Injection pressure [kPa] [atm] [psi]
Po Standard pressure [kPa] [atm] [psi]
PI Pressure in carburetor throat [kPa] [atm] [psi]
Pv Vapor pressure [kPa] [atm] [psi]
Q Heat transfer [kJ] [BTU]
Q Heat transfer rate [kW] [hp] [BTU/sec]
QHHV Higher heating value [kJ/kg] [BTU/lbm]
QHV Heating value of fuel [kJ/kg] [BTU/lbm]
QLHV Lower heating value [kJ/kg] [BTU/lbm]
R Ratio of connecting rod length to crank offset
R Gas constant [kJ/kg-K] [ft-Ibf/lbm-OR] [BTU/lbm-OR]
Re Reynolds number RON Research octane number S Stroke length [cm] [in.]
Sg Specific gravity
Notation xix
W Specific work [kJ/kg] [ft-Ibf/lbm] [BTU/lbm]
Wb Brake-specific work [kJ/kg] [ft-Ibf/lbm] [BTU/lbm]
wf Friction-specific work [kJ/kg] [ft-Ibf/lbm] [BTU/lbm]
Wi Indicated-specific work [kJ/kg] [ft-Ibf/lbm] [BTU/lbm]
x Distance [em] [m] [in.] [ft]
Xex Fraction of exhaust Xr Exhaust residual
Xv Mole fraction of water vapor a Pressure ratio
a Ratio of valve areas 13 Cutoff ratio
r
Angular momentum [kg-m2/sec] [lbm-ft2/sec]eg Emissivity of gas ew Emissivity of wall
T]c Combustion efficiency
[%]
T]f Fuel conversion efficiency
[%]
T]m Mechanical efficiency
[%]
T]s Isentropic efficiency
[%]
T]t Thermal efficiency
[%]
T]v Volumetric efficiency of the engine
[%]
9 Crank angle measured from TDC [0]
Ace Charging efficiency
Adr Delivery ratio
Arc Relative charge
Ase Scavenging efficiency
Ate Trapping efficiency
/.L Dynamic viscosity [kg/m-sec] [lbm/ft-sec]
/.Lg Dynamic viscosity of gas [kg/m-sec] [lbm/ft -see]
v Stoichiometric coefficients
P Density [kg/m3 ] [lbm/ft3 ] Pa Density of air [kg/m3 ] [lbm/ft3 ]
Po Density of air at standard conditions [kg/m3 ] [lbm/ft3 ] Pf Density of fuel [kg/m3 ] [lbm/ft3 ]
CJ' Stefan-Boltzmann constant [W/m2-K4] [BTU/hr-ft2-OR4]
T Torque [N-m] [lbf-ft]
Ts Shear force per unit area [N/m2] [lbf/ft2 ]
<I> Equivalence ratio
<I> Angle between connecting rod and centerline of the cylinder w Angular velocity of swirl [rev/see]
Wv Specific humidity [kgv/kga] [grainsv/lbma]
1
Introduction
This chapter introduces and defines the internal combustion engine. It lists ways of classifying engines and terminology used in engine technology. Descriptions are given of many common engine components and of basic four-stroke and two-stroke cycles for both spark ignition and compression ignition engines.
1-1 INTRODUCTION
The internal combustion engine (Ie) is a heat engine that converts chemical energy in a fuel into mechanical energy, usually made available on a rotating output shaft.
Chemical energy of the fuel is first converted to thermal energy by means of com- bustion or oxidation with air inside the engine. This thermal energy raises the temperature and pressure of the gases within the engine, and the high-pressure gas then expands against the mechanical mechanisms of the engine. This expansion is converted by the mechanical linkages of the engine to a rotating crankshaft, which is the output of the engine. The crankshaft, in turn, is connected to a transmission and/or power train to transmit the rotating mechanical energy to the desired final use. For engines this will often be the propulsion of a vehicle (i.e., automobile, truck, locomotive, marine vessel, or airplane). Other applications include stationary
1
2 Introduction Chap. 1 engines to drive generators or pumps, and portable engines for things like chain saws and lawn mowers.
Most internal combustion engines are reciprocating engines having pistons that reciprocate back and forth in cylinders internally within the engine. This book concentrates on the thermodynamic study of this type of engine. Other types of IC engines also exist in much fewer numbers, one important one being the rotary engine [104]. These engines will be given brief coverage. Engine types not covered by this book include steam engines and gas turbine engines, which are better classi- fied as external combustion engines (i.e., combustion takes place outside the mechanical engine system). Also not included in this book, but which could be clas- sified as internal combustion engines, are rocket engines, jet engines, and firearms.
Reciprocating engines can have one cylinder or many, up to 20 or more. The cylinders can be arranged in many different geometric configurations. Sizes range from small model airplane engines with power output on the order of 100 watts to large multicylinder stationary engines that produce thousands of kilowatts per cylinder.
There are so many different engine manufacturers, past, present, and future, that produce and have produced engines which differ in size, geometry, style, and operating characteristics that no absolute limit can be stated for any range of engine characteristics (i.e., size, number of cylinders, strokes in a cycle, etc.). This book will work within normal characteristic ranges of engine geometries and operating para- meters, but there can always be exceptions to these.
Early development of modern internal combustion engines occurred in the lat- ter half of the 1800s and coincided with the development of the automobile. History records earlier examples of crude internal combustion engines and self-propelled road vehicles dating back as far as the 1600s [29]. Most of these early vehicles were steam-driven prototypes which never became practical operating vehicles. Technol- ogy, roads, materials, and fuels were not yet developed enough. Very early examples of heat engines, including both internal combustion and external combustion, used gun powder and other solid, liquid, and gaseous fuels. Major development of the modern steam engine and, consequently, the railroad locomotive occurred in the lat- ter half of the 1700s and early 1800s. By the 1820s and 1830s, railroads were present in several countries around the world.
HISTORIC-ATMOSPHERIC ENGINES
Most of the very earliest internal combustion engines of the 17th and 18th centuries can be classified asatmospheric engines. These were large engines with a single piston and cylinder, the cylinder being open on the end. Combustion was initiated in the open cylinder using any of the various fuels which were available. Gunpowder was often used as the fuel. Immediately after combustion, the cylinder would be full of hot exhaust gas at atmospheric pressure. At this time, the cylinder end was closed and the trapped gas was allowed to cool. As the gas cooled, it cre-
Figure 1-1 The Charter Engine made in 1893 at the Beloit works of Fairbanks, Morse &Company was one of the first successful gasoline engine offered for sale in the United States. Printed with permission, Fairbanks Morse Engine Division, Coltec Industries.
ated a vacuum within the cylinder. This caused a pressure differential across the piston, atmospheric pressure on one side and a vacuum on the other. As the piston moved because of this pressure differential, it would do work by being connected to an external system, such as raising a weight [29].
Some early steam engines also were atmospheric engines. Instead of combustion, the open cylinder was filled with hot steam. The end was then closed and the steam was allowed to cool and condense. This cre- ated the necessary vacuum.
In addition to a great amount of experimentation and development in Europe and the United States during the middle and latter half of the 1800s, two other tech- nological occurrences during this time stimulated the emergence of the internal combustion engine. In 1859, the discovery of crude oil in Pennsylvania finally made available the development of reliable fuels which could be used in these newly developed engines. Up to this time, the lack of good, consistent fuels was a major drawback in engine development. Fuels like whale oil, coal gas, mineral oils, coal, and gun powder which were available before this time were less than ideal for engine use and development. It still took many years before products of the petro- leum industry evolved from the first crude oil to gasoline, the automobile fuel of the 20th century. However, improved hydrocarbon products began to appear as early
Figure 1·2 Ford Taurus SHO 3.4 liter (208 in.3), spark ignition, four-stroke cycle engine. The engine is rated at 179 kW at 6500 RPM (240 hp) and develops 305 N-m of torque at 4800 RPM (225Ibf-ft). It is a 60° V8 with 8.20 cm bore (3.23 in.), 7.95 cm stroke (3.13 in.), and a compression ratio of 10: 1. The engine has four chain driven camshafts mounted in aluminum heads with four valves per cylinder and coil-on- plug ignition. Each spark plug has a separate high-voltage coil and is fired by Ford's Electronic Distributorless Ignition System (ED IS). Courtesy of Ford Motor Company.
as the 1860s and gasoline, lubricating oils, and the internal combustion engine evolved together.
The second technological invention that stimulated the development of the internal combustion engine was the pneumatic rubber tire, which was first marketed by John B. Dunlop in 1888 [141]. This invention made the automobile much more practical and desirable and thus generated a large market for propulsion systems, including the internal combustion engine.
During the early years of the automobile, the internal combustion engine com- peted with electricity and steam engines as the basic means of propulsion. Early in the 20th century, electricity and steam faded from the automobile picture-electricity because of the limited range it provided, and steam because of the long start-up time needed. Thus, the 20th century is the period of the internal combustion engine and
Sec. 1-3 Engine Classifications 5 the automobile powered by the internal combustion engine. Now, at the end of the century, the internal combustion engine is again being challenged by electricity and other forms of propulsion systems for automobiles and other applications. What goes around comes around.
1-2 EARLY HISTORY
During the second half of the 19th century, many different styles of internal com- bustion engines were built and tested. Reference [29] is suggested as a good history of this period. These engines operated with variable success and dependability using many different mechanical systems and engine cycles.
The first fairly practical engine was invented by J.J.E. Lenoir (1822-1900) and appeared on the scene about 1860 (Fig. 3-19). During the next decade, several hun- dred of these engines were built with power up to about 4.5 kW (6 hp) and mechanical efficiency up to 5 %. The Lenoir engine cycle is described in Section 3-13. In 1867 the Otto-Langen engine, with efficiency improved to about 11%,was first introduced, and several thousand of these were produced during the next decade. This was a type of atmospheric engine with the power stroke propelled by atmospheric pressure acting against a vacuum. Nicolaus A. Otto (1832-1891) and Eugen Langen (1833-1895) were two of many engine inventors of this period.
During this time, engines operating on the same basic four-stroke cycle as the modern automobile engine began to evolve as the best design. Although many peo- ple were working on four-stroke cycle design, Otto was given credit when his prototype engine was built in 1876.
In the 1880s the internal combustion engine first appeared in automobiles [45].
Also in this decade the two-stroke cycle engine became practical and was manufac- tured in large numbers.
By 1892, Rudolf Diesel (1858-1913) had perfected his compression ignition engine into basically the same diesel engine known today. This was after years of development work which included the use of solid fuel in his early experimental engines. Early compression ignition engines were noisy, large, slow, single-cylinder engines. They were, however, generally more efficient than spark ignition engines. It wasn't until the 1920s that multicylinder compression ignition engines were made small enough to be used with automobiles and trucks.
1-3 ENGINE CLASSIFICATIONS
Internal combustion engines can be classified in a number of different ways:
1. Types of Ignition
(a) Spark Ignition (SI). An SI engine starts the combustion process in each cycle by use of a spark plug. The spark plug gives a high-voltage electrical
Figure 1-3 1955 Chevrolet "small block" V8 engine with 265 in.3(4.34 L) displace- ment. The four-stroke cycle, spark ignition engine was equipped with a carburetor and overhead valves. Copyright General Motors Corp., used with permission.
discharge between two electrodes which ignites the air-fuel mixture in the combustion chamber surrounding the plug. In early engine development, before the invention of the electric spark plug, many forms of torch holes were used to initiate combustion from an external flame.
(b) Compression Ignition (CI). The combustion process in a CI engine starts when the air-fuel mixture self-ignites due to high temperature in the com- bustion chamber caused by high compression.
2. Engine Cycle
(a) Four-Stroke Cycle. A four-stroke cycle experiences four piston move- ments over two engine revolutions for each cycle.
(b) Two-Stroke Cycle.A two-stroke cycle has two piston movements over one revolution for each cycle.
Figure 1-4 Engine Classification by Valve Location. (a)Valve in block, L head.
Older automobiles and some small engines. (b) Valve in head, I head. Standard on modern automobiles. (c) One valve in head and one valve in block, F head. Older, less common automobiles. (d)Valves in block on opposite sides of cylinder, T head.
Some historic automobile engines.
Three-stroke cycles and six-stroke cycles were also tried in early engine devel- opment [29].
3. Valve Location (see Fig. 1-4)
(a) Valves in head (overhead valve),also called I Head engine.
(b) Valves in block (flat head), also called L Head engine. Some historic engines with valves in block had the intake valve on one side of the cylin- der and the exhaust valve on the other side. These were called T Head engines.
PRINCIPAL FLATU:E" OF THE KNlGHT ENGINE
The Valve Functions Are Performed by Two Concentric, Ported Sleeves, Generally of Cast Iron, Which Are Inserted between the Cylinder-Wall and the Piston. The Sleeves Are Given a Reciprocat- ing Motion by Connection to an Eccentric Shaft Driven from the Crankshaft through the Usual 2 to 1 Gear, Their Stroke in the Older Designs at Least, Being Either 1 or 1v.. In. The' Sleeves Project from the Cylinder at the Bottom and, at the Top They Exte!,d into an Annular Space between the Cylinder-Wall a'nd the SpeCial Form of Cylinder-Head So That, during the Compression and the Power Strokes, the Gases Do Not Come Into Contact with the Cylinder-Wall But Are Separated Therefrom by Two Layers of Cast Iron and Two Films of Lubricating Oil. The Cylinder, As Well As Each Sleeve, Is Provided with an Exhaust-Port on One Side and with an Inlet-Port on the Opposite Side. The Passage for Either the Inlet or the Exhaust Is Open When All Three of th€.
Ports on the Particular Side Are In Register with Each Other
Figure 1-5 Sectional view of Willy-Knight sleeve valve engine of 1926. Reprinted with permission from © 1995 Automotive Engineering magazine. Society of Auto- motive Engineers, Inc.
(c) One valve in head (usually intake) and one in block, also called FHead engine;this is much less common.
4. Basic Design
(a) Reciprocating. Engine has one or more cylinders in which pistons recipro- cate back and forth. The combustion chamber is located in the closed end of each cylinder. Power is delivered to a rotating output crankshaft by mechanical linkage with the pistons.
Figure 1·6 Chevrolet LT4 V8, four-stroke cycle, spark ignition engine with 5.7liter displacement. This fuel-injected, overhead valve engine was an option in the 1986 Corvette. Copyright General Motors Corp., used with permission.
(b) Rotary. Engine is made of a block (stator) built around a large non-con- centric rotor and crankshaft. The combustion chambers are built into the nonrotating block.
5. Position and Number of Cylinders of Reciprocating Engines (Fig. 1-7) (a) Single Cylinder. Engine has one cylinder and piston connected to the
crankshaft.
(b) In-Line. Cylinders are positioned in a straight line, one behind the other along the length of the crankshaft. They can consist of 2 to 11 cylinders or possibly more. In-line four-cylinder engines are very common for automo- bile and other applications. In-line six and eight cylinders are historically common automobile engines. In-line engines are sometimes calledstraight (e.g., straight six or straight eight).
(c) V Engine. Two banks of cylinders at an angle with each other along a sin- gle crankshaft. The angle between the banks of cylinders can be anywhere from 15° to 120°, with 60°-90° being common. V engines have even num- bers of cylinders from 2 to 20 or more. V6s and V8s are common automobile engines, with V12s and V16s (historic) found in some luxury and high-performance vehicles.
(d) Opposed Cylinder Engine. Two banks of cylinders opposite each other on a single crankshaft (a V engine with a 180° V). These are common on small
Sec. 1-3 Engine Classifications 11 aircraft and some automobiles with an even number of cylinders from two to eight or more. These engines are often called flat engines (e.g., flat four).
(e) W Engine. Same as a V engine except with three banks of cylinders on the same crankshaft. Not common, but some have been developed for racing automobiles, both modern and historic. Usually 12 cylinders with about a 60°angle between each bank.
(1) Opposed Piston Engine. Two pistons in each cylinder with the combustion chamber in the center between the pistons. A single-combustion process causes two power strokes at the same time, with each piston being pushed away from the center and delivering power to a separate crankshaft at each end of the cylinder. Engine output is either on two rotating crankshafts or on one crankshaft incorporating complex mechanical linkage.
(g) Radial Engine. Engine with pistons positioned in a circular plane around the central crankshaft. The connecting rods of the pistons are connected to a master rod which, in turn, is connected to the crankshaft. A bank of cylin- ders on a radial engine always has an odd number of cylinders ranging from 3 to 13 or more. Operating on a four-stroke cycle, every other cylin- der fires and has a power stroke as the crankshaft rotates, giving a smooth operation. Many medium- and large-size propeller-driven aircraft use radial engines. For large aircraft, two or more banks of cylinders are mounted together, one behind the other on a single crankshaft, making one powerful, smooth engine. Very large ship engines exist with up to 54 cylinders, six banks of 9 cylinders each.
HISTORIC-RADIAL ENGINES
There are at least two historic examples of radial engines being mounted with the crankshaft fastened to the vehicle while the heavy bank of radial cylinders rotated around the stationary crank. The Sopwith Camel, a very successful World War I fighter aircraft, had the engine so mounted with the propeller fastened to the rotating bank of cylinders. The gyroscopic forces generated by the large rotating engine mass allowed these planes to do some maneuvers which were not possible with other airplanes, and restricted them from some other maneuvers. Snoopy has been flying a Sopwith Camel in his battles with the Red Baron for many years.
The little-known early Adams-Farwell automobiles had three- and five-cylinder radial engines rotating in a horizontal plane with the station- ary crankshaft mounted vertically. The gyroscopic effects must have given these automobiles very unique steering characteristics [45].
Figure 1-8 Supercharger used to increase inlet air pressure to engine. Compressor is driven off engine crankshaft, which gives fast response to speed changes but adds parasitic load to engine.
6. Air Intake Process
(a) Naturally Aspirated. No intake air pressure boost system.
(b) Supercharged. Intake air pressure increased with the compressor driven off of the engine crankshaft (Fig. 1-8).
(c) Turbocharged. Intake air pressure increased with the turbine-compressor driven by the engine exhaust gases (Fig. 1-9).
(d) Crankcase Compressed. Two-stroke cycle engine which uses the crankcase as the intake air compressor. Limited development work has also been done on design and construction of four-stroke cycle engines with crankcase compression.
7. Method of Fuel Input for SI Engines (a) Carbureted.
(b) Multipoint Port Fuel Injection. One or more injectors at each cylinder intake.
(c) Throttle Body Fuel Injection. Injectors upstream in intake manifold.
8. Fuel Used (a) Gasoline.
(b) Diesel Oil or Fuel Oil.
(c) Gas, Natural Gas, Methane.
(d) LPG.
(e) Alcohol-Ethyl, Methyl.
(f) Dual Fuel. There are a number of engines that use a combination of two or more fuels. Some, usually large, CI engines use a combination of methane and diesel fuel. These are attractive in developing third-world countries because of the high cost of diesel fuel. Combined gasoline-alcohol fuels
Figure 1-9 Turbocharger used to increase inlet air pressure to engine. Turbine that drives compressor is powered by exhaust flow from engine. This adds no load to the engine but results in turbo lag, a slower response to engine speed changes.
are becoming more common as an alternative to straight gasoline automo- bile engine fuel.
(g) Gasohol. Common fuel consisting of 90% gasoline and 10% alcohol.
9. Application
(a) Automobile, Truck, Bus.
(b) Locomotive.
(c) Stationary.
(d) Marine.
(e) Aircraft.
(f) Small Portable, Chain Saw, Model Airplane.
LO. Type of Cooling (a) Air Cooled.
(b) Liquid Cooled, Water Cooled.
Several or all of these classifications can be used at the same time to identify a given engine. Thus, a modern engine might be called a turbocharged, reciprocating, spark ignition, four-stroke cycle, overhead valve, water-cooled, gasoline, multipoint fuel-injected, V8 automobile engine.
Figure 1-10 General Motors 7.4 liter L29, V8, four-stroke cycle, spark ignition, truck engine. Displacement is 454 in.3(7.44 L) with 4.25 in. bore (10.80 cm) and 4.00 in. stroke (10.16 cm). The engine has a maximum speed of 5000 RPM, with a com- pression ratio of 9.0: 1, overhead valves, and multipoint port fuel injection. This engine was used in several models of 1996 Chevrolet and GMC trucks. Copyright General Motors Corp., used with permission.
'-4 TERMINOLOGY AND ABBREVIATIONS
The following terms and abbreviations are commonly used in engine technology lit- erature and will be used throughout this book. These should be learned to assure maximum understanding of the following chapters.
Internal Combustion (Ie)
Spark Ignition (81) An engine in which the combustion process in each cycle is started by use of a spark plug.
Compression Ignition (CI) An engine in which the combustion process starts when the air-fuel mixture self-ignites due to high temperature in the combustion chamber caused by high compression. CI engines are often called Diesel engines, especially in the non-technical community.
Figure 1-11 Power and torque curves of GM 7.4 liter L29 truck engine shown in Fig. 1-10. The engine has a power rating of 290 hp (216 kW) at 4200 RPM and a torque rating of 410 lbf-ft (556 N-m) at 3200 RPM. Copyright General Motors Corp., used with permission.
Top-Dead-Center (TDC) Position of the piston when it stops at the furthest point away from the crankshaft. Top because this position is at the top of most engines (not always), and dead because the piston stops at this point. Because in some engines top-de ad-center is not at the top of the engine (e.g., horizon- tally opposed engines, radial engines, etc.), some Sources call this position Head-End-Dead-Center (HEDC). Some sources call this position Top-Center (TC). When an occurrence in a cycle happens before TDC, it is often abbrevi- ated bTDC or bTe. When the occurrence happens after TDC, it will be abbreviated aTDC or aTe. When the piston is at TDC, the volume in the cylinder is a minimum called the clearance volume.
Bottom-Dead-Center (BDC) Position of the piston when it stops at the point clos- est to the crankshaft. Some sources call thisCrank-End-Dead-Center (CEDC) because it is not always at the bottom of the engine. Some sources call this point Bottom-Center (BC). During an engine cycle things can happen before bottom-dead-center, bBDC or bBC, and after bottom-de ad-center, aBDC or aBe.
Direct Injection (DI) Fuel injection into the main combustion chamber of an engine. Engines have either one main combustion chamber (open chamber)
Introduction Chap. 1
Figure 1-12 Poppet valve is spring loaded closed, and pushed open by cam action at proper time in cycle. Most auto- mobile engines and other reciprocating engines use poppet valves. Much less common are sleeve valves and rotary valves. Components include: (A) valve seat, (B) head, (C) stem, (D) guide, (E) spring, (F) camshaft, (G) manifold.
or a divided combustion chamber made up of a main chamber and a smaller connected secondary chamber.
Indirect Injection (IDI) Fuel injection into the secondary chamber of an engine with a divided combustion chamber.
Bore Diameter of the cylinder or diameter of the piston face, which is the same minus a very small clearance.
Stroke Movement distance of the piston from one extreme position to the other:
TDC to BDC or BDC to TDC.
Clearance Volume Minimum volume in the combustion chamber with piston at TDC.
Displacement or Displacement Volume Volume displaced by the piston as it trav- els through one stroke. Displacement can be given for one cylinder or for the entire engine (one cylinder times number of cylinders). Some literature calls thisswept volume.
Smart Engine Engine with computer controls that regulate operating characteris- tics such as air-fuel ratio, ignition timing, valve timing, exhaust control, intake tuning, etc. Computer inputs come from electronic, mechanical, thermal, and chemical sensors located throughout the engine. Computers in some automo-
Sec. 1-4 Terminology and Abbreviations 17 biles are even programmed to adjust engine operation for things like valve wear and combustion chamber deposit buildup as the engine ages. In automobiles the same computers are used to make smart carsby controlling the steering, brakes, exhaust system, suspension, seats, anti-theft systems, sound-entertain- ment systems, shifting, doors, repair analysis, navigation, noise suppression, environment, comfort, etc. On some systems engine speed is adjusted at the instant when the transmission shifts gears, resulting in a smoother shifting process. At least one automobile model even adjusts this process for transmis- sion fluid temperature to assure smooth shifting at cold startup.
Engine Management System (EMS) Computer and electronics used to control smart engines.
Wide-Open Throttle (WOT) Engine operated with throttle valve fully open when maximum power and/or speed is desired.
Ignition Delay (ID) Time interval between ignition initiation and the actual start of combustion.
Figure 1-13 Harley-Davidson two-cylinder, air-cooled, overhead valve "Knuckle- head" motorcycle engine first introduced in 1936. The 45° V engine had displacement of 60 cubic inches with 3.3125 inch bore and 3.500 inch stroke. Operating on a four- stroke cycle with a compression ratio of 7: 1 the engine was rated at 40 bhp at 4800 RPM. Ignition was by Harley-Davidson generator-battery system. Photograph cour- tesy of the Harley-Davidson Juneau A venue Archives. All rights reserved. Copyright Harley-Davidson.
Figure 1-14 Harley-Davidson motorcycle of 1936 powered by "Knucklehead"
engine shown in Fig. 1-13. The motorcycle had a rated top speed of 90-95 MPH with a fuel economy of 35-50 MPG. Photograph courtesy of the Harley-Davidson Juneau Avenue Archives. All rights reserved. Copyright Harley-Davidson.
Air-Fuel Ratio (AF) Ratio of mass of air to mass of fuel input into engine.
Fuel-Air Ratio (FA) Ratio of mass of fuel to mass of air input into engine.
Brake Maximum Torque (BMT) Speed at which maximum torque occurs.
Overhead Valve (ORV) Valves mounted in engine head.
Overhead Cam (aRC) Camshaft mounted in engine head, giving more direct con- trol of valves which are also mounted in engine head.
Fuel Injected (FI)
'-5 ENGINE COMPONENTS
The following is a list of major components found in most reciprocating internal combustion engines (see Fig. 1-15).
Block Body of engine containing the cylinders, made of cast iron or aluminum. In many older engines, the valves and valve ports were contained in the block.
The block of water-cooled engines includes a water jacket cast around the cylinders. On air-cooled engines, the exterior surface of the block has cooling fins.
Camshaft Rotating shaft used to push open valves at the proper time in the engine cycle, either directly or through mechanical or hydraulic linkage (push rods,
Figure 1-15 Cross-section of four-stroke cycle S1 engine showing engine compo- nents; (A) block, (B) camshaft, (C) combustion chamber, (D) connecting rod, (E) crankcase, (F) crankshaft, (G) cylinder, (H) exhaust manifold, (I) head, (J) intake manifold, (K) oil pan, (L) piston, (M) piston rings, (N) push rod, (0) spark plug, (P) valve,(Q)water jacket.
rocker arms, tappets). Most modern automobile engines have one or more camshafts mounted in the engine head (overhead cam). Most older engines had camshafts in the crankcase. Camshafts are generally made of forged steel or cast iron and are driven off the crankshaft by means of a belt or chain (tim- ing chain). To reduce weight, some cams are made from a hollow shaft with
20 Introduction Chap.1 the cam lobes press-fit on. In four-stroke cycle engines, the camshaft rotates at half engine speed.
Carburetor Venturi flow device which meters the proper amount of fuel into the air flow by means of a pressure differential. For many decades it was the basic fuel metering system on all automobile (and other) engines. It is still used on low- cost small engines like lawn mowers, but is uncommon on new automobiles.
Catalytic converter Chamber mounted in exhaust flow containing catalytic mater- ial that promotes reduction of emissions by chemical reaction.
Combustion chamber The end of the cylinder between the head and the piston face where combustion occurs. The size of the combustion chamber continuously changes from a minimum volume when the piston is at TDC to a maximum when the piston is at BDC. The term "cylinder" is sometimes synonymous with
"combustion chamber" (e.g., "the engine was firing on all cylinders"). Some engines have open combustion chambers which consist of one chamber for each cylinder. Other engines have divided chambers which consist of dual chambers on each cylinder connected by an orifice passage.
Connecting rod Rod connecting the piston with the rotating crankshaft, usually made of steel or alloy forging in most engines but may be aluminum in some small engines.
Connecting rod bearing Bearing where connecting rod fastens to crankshaft.
Cooling fins Metal fins on the outside surfaces of cylinders and head of an air- cooled engine. These extended surfaces cool the cylinders by conduction and convection.
Crankcase Part of the engine block surrounding the rotating crankshaft. In many engines, the oil pan makes up part of the crankcase housing.
Crankshaft Rotating shaft through which engine work output is supplied to exter- nal systems. The crankshaft is connected to the engine block with the main bearings. It is rotated by the reciprocating pistons through connecting rods connected to the crankshaft, offset from the axis of rotation. This offset is sometimes called crank throw or crank radius. Most crankshafts are made of forged steel, while some are made of cast iron.
Cylinders The circular cylinders in the engine block in which the pistons recipro- cate back and forth. The walls of the cylinder have highly polished hard surfaces. Cylinders may be machined directly in the engine block, or a hard metal (drawn steel) sleeve may be pressed into the softer metal block. Sleeves may be dry sleeves, which do not contact the liquid in the water jacket, or wet sleeves, which form part of the water jacket. In a few engines, the cylinder walls are given a knurled surface to help hold a lubricant film on the walls. In some very rare cases, the cross section of the cylinder is not round.
Sec. 1-5 Engine Components 21 Exhaust manifold Piping system which carries exhaust gases away from the engine
cylinders, usually made of cast iron.
Exhaust system Flow system for removing exhaust gases from the cylinders, treat- ing them, and exhausting them to the surroundings. It consists of an exhaust manifold which carries the exhaust gases away from the engine, a thermal or catalytic converter to reduce emissions, a muffler to reduce engine noise, and a tailpipe to carry the exhaust gases away from the passenger compartment.
Fan Most engines have an engine-driven fan to increase air flow through the radi- ator and through the engine compartment, which increases waste heat removal from the engine. Fans can be driven mechanically or electrically, and can run continuously or be used only when needed.
Flywheel Rotating mass with a large moment of inertia connected to the crank- shaft of the engine. The purpose of the flywheel is to store energy and furnish a large angular momentum that keeps the engine rotating between power strokes and smooths out engine operation. On some aircraft engines the pro- peller serves as the flywheel, as does the rotating blade on many lawn mowers.
Fuel injector A pressurized nozzle that sprays fuel into the incoming air on SI engines or into the cylinder on CI engines. On SI engines, fuel injectors are located at the intake valve ports on multipoint port injector systems and upstream at the intake manifold inlet on throttle body injector systems. In a few SI engines, injectors spray directly into the combustion chamber.
Fuel pump Electrically or mechanically driven pump to supply fuel from the fuel tank (reservoir) to the engine. Many modern automobiles have an electric fuel pump mounted submerged in the fuel tank. Some small engines and early automobiles had no fuel pump, relying on gravity feed.
HISTORIC-FUEL PUMPS
Lacking a fuel pump, it was necessary to back Model T Fords (1909-1927) up high-slope hills becauseofthelocation ofthe fuel tank rel- ative to the engine.
Glow plug Small electrical resistance heater mounted inside the combustion cham- ber of many CI engines, used to preheat the chamber enough so that combustion will occur when first starting a cold engine. The glow plug is turned off after the engine is started.
Head The piece which closes the end of the cylinders, usually containing part of the clearance volume of the combustion chamber. The head is usually cast iron or aluminum, and bolts to the engine block. In some less common engines, the
22 Introduction Chap. 1 head is one piece with the block. The head contains the spark plugs in SI engines and the fuel injectors in CI engines and some SI engines. Most modern engines have the valves in the head, and many have the camshaft(s) positioned there also (overhead valves and overhead cam).
Head gasket Gasket which serves as a sealant between the engine block and head where they bolt together. They are usually made in sandwich construction of metal and composite materials. Some engines use liquid head gaskets.
Intake manifold Piping system which delivers incoming air to the cylinders, usually made of cast metal, plastic, or composite material. In most SI engines, fuel is added to the air in the intake manifold system either by fuel injectors or with a carburetor. Some intake manifolds are heated to enhance fuel evaporation.
The individual pipe to a single cylinder is called arunner.
Main bearing The bearings connected to the engine block in which the crankshaft rotates. The maximum number of main bearings would be equal to the number of pistons plus one, or one between each set of pistons plus the two ends. On some less powerful engines, the number of main bearings is less than this maximum.
Oil pan Oil reservoir usually bolted to the bottom of the engine block, making up part of the crankcase. Acts as the oil sump for most engines.
Oil pump Pump used to distribute oil from the oil sump to required lubrication points. The oil pump can be electrically driven, but is most commonly mechan- ically driven by the engine. Some small engines do not have an oil pump and are lubricated by splash distribution.
Oil sump Reservoir for the oil system of the engine, commonly part of the crankcase. Some engines (aircraft) have a separate closed reservoir called a dry sump.
Piston The cylindrical-shaped mass that reciprocates back and forth in the cylin- der, transmitting the pressure forces in the combustion chamber to the rotating crankshaft. The top of the piston is called the crown and the sides are called the skirt. The face on the crown makes up one wall of the combustion chamber and may be a flat or highly contoured surface. Some pistons contain an indented bowl in the crown, which makes up a large percent of the clearance volume. Pistons are made of cast iron, steel, or aluminum. Iron and steel pis- tons can have sharper corners because of their higher strength. They also have lower thermal expansion, which allows for tighter tolerances and less crevice volume. Aluminum pistons are lighter and have less mass inertia. Sometimes synthetic or composite materials are used for the body of the piston, with only the crown made of metal. Some pistons have a ceramic coating on the face.
Piston rings Metal rings that fit into circumferential grooves around the piston and form a sliding surface against the cylinder walls. Near the top of the piston are
Sec. 1-5 Engine Components 23 usually two or more compression rings made of highly polished hard chrome steel. The purpose of these is to form a seal between the piston and cylinder walls and to restrict the high-pressure gases in the combustion chamber from leaking past the piston into the crankcase (blowby). Below the compression rings on the piston is at least one oil ring, which assists in lubricating the cylin- der walls and scrapes away excess oil to reduce oil consumption.
Push rods Mechanical linkage between the camshaft and valves on overhead valve engines with the camshaft in the crankcase. Many push rods have oil passages through their length as part of a pressurized lubrication system.
Radiator Liquid-to-air heat exchanger of honeycomb construction used to remove heat from the engine coolant after the engine has been cooled. The radiator is usually mounted in front of the engine in the flow of air as the automobile moves forward. An engine-driven fan is often used to increase air flow through the radiator.
Spark plug Electrical device used to initiate combustion in an SI engine by creat- ing a high-voltage discharge across an electrode gap. Spark plugs are usually made of metal surrounded with ceramic insulation. Some modern spark plugs have built-in pressure sensors which supply one of the inputs into engine control.
Speed control-cruise control Automatic electric-mechanical control system that keeps the automobile operating at a constant speed by controlling engine speed.
Starter Several methods are used to start IC engines. Most are started by use of an electric motor (starter) geared to the engine flywheel. Energy is supplied from an electric battery.
On some very large engines, such as those found in large tractors and con- struction equipment, electric starters have inadequate power, and small IC engines are used as starters for the large IC engines. First the small engine is started with the normal electric motor, and then the small engine engages gear- ing on the flywheel of the large engine, turning it until the large engine starts.
Early aircraft engines were often started by hand spinning the propeller, which also served as the engine flywheel. Many small engines on lawn mowers and similar equipment are hand started by pulling a rope wrapped around a pulley connected to the crankshaft.
Compressed air is used to start some large engines. Cylinder release valves are opened, which keeps the pressure from increasing in the compres- sion strokes. Compressed air is then introduced into the cylinders, which rotates the engine in afree-wheeling mode. When rotating inertia is estab- lished, the release valves are closed and the engine is fired.
24
Introduction Chap. 1
HISTORIC-ST ARTERS
Early automobile engines were started with hand cranks that connected with the crankshaft of the engine. This was a difficult and dan- gerous process, sometimes resulting in broken fingers and arms when the engine would fire and snap back the hand crank. The first electric starters appeared on the 1912Cadillac automobiles, invented by C. Kettering, who was motivated when his friend was killed in the process of hand starting an automobile [45].
Supercharger Mechanical compressor powered off of the crankshaft, used to com- press incoming air of the engine.
Throttle Butterfly valve mounted at the upstream end of the intake system, used to control the amount of air flow into an SI engine. Some small engines and stationary constant-speed engines have no throttle.
Turbocharger Turbine-compressor used to compress incoming air into the engine.
The turbine is powered by the exhaust flow of the engine and thus takes very little useful work from the engine.
Valves Used to allow flow into and out of the cylinder at the proper time in the cycle. Most engines use poppet valves, which are spring loaded closed and pushed open by camshaft action (Fig. 1-12). Valves are mostly made of forged steel. Surfaces against which valves close are called valve seats and are made of hardened steel or ceramic. Rotary valvesandsleeve valvesare sometimes used, but are much less common. Many two-stroke cycle engines haveports (slots) in the side of the cylinder walls instead of mechanical valves.
Water jacket System of liquid flow passages surrounding the cylinders, usually constructed as part of the engine block and head. Engine coolant flows through the water jacket and keeps the cylinder walls from overheating. The coolant is usually a water-ethylene glycol mixture.
Water pump Pump used to circulate engine coolant through the engine and radia- tor. It is usually mechanically run off of the engine.
Wrist pin Pin fastening the connecting rod to the piston (also called thepiston pin).
1-6 BASIC ENGINE CYCLES
Most internal combustion engines, both spark ignition and compression ignition, operate on either a four-stroke cycle or a two-stroke cycle. These basic cycles are fairly standard for all engines, with only slight variations found in individual designs
Sec. 1-6 Basic Engine Cycles 25 Four-Stroke SI Engine Cycle (Fig. 1-16)
1. First Stroke: Intake Stroke or Induction The piston travels from TDC to BDC with the intake valve open and exhaust valve closed. This creates an increasing volume in the combustion chamber, which in turn creates a vacuum. The resulting pressure differential through the intake system from atmospheric pressure on the outside to the vacuum on the inside causes air to be pushed into the cylinder. As the air passes through the intake system, fuel is added to it in the desired amount by means of fuel injectors or a carburetor.
2. Second Stroke: Compression Stroke When the piston reaches BDC, the intake valve closes and the piston travels back to TDC with all valves closed. This compresses the air-fuel mixture, raising both the pressure and temperature in the cylinder. The finite time required to close the intake valve means that actual com- pression doesn't start until sometime aBDC. Near the end of the compression stroke, the spark plug is fired and combustion is initiated.
3. Combustion Combustion of the air-fuel mixture occurs in a very short but finite length of time with the piston near TDC (i.e., nearly constant-volume com- bustion). It starts near the end of the compression stroke slightly bTDC and lasts into the power stroke slightly aTDC. Combustion changes the composition of the gas mixture to that of exhaust products and increases the temperature in the cylin- der to a very high peak value. This, in turn, raises the pressure in the cylinder to a very high peak value.
4. Third Stroke: Expansion Stroke or Power Stroke With all valves closed, the high pressure created by the combustion process pushes the piston away from TDC. This is the stroke which produces the work output of the engine cycle. As the piston travels from TDC to BDC, cylinder volume is increased, causing pressure and temperature to drop.
5. Exhaust Blowdown Late in the power stroke, the exhaust valve is opened and exhaust blow down occurs. Pressure and temperature in the cylinder are still high relative to the surroundings at this point, and a pressure differential is created through the exhaust system which is open to atmospheric pressure. This pressure differential causes much of the hot exhaust gas to be pushed out of the cylinder and through the exhaust system when the piston is near BDC. This exhaust gas carries away a high amount of enthalpy, which lowers the cycle thermal efficiency. Opening the exhaust valve before BDC reduces the work obtained during the power stroke but is required because of the finite time needed for exhaust blowdown.
6. Fourth Stroke: Exhaust Stroke By the time the piston reaches BDC, exhaust blowdown is complete, but the cylinder is still full of exhaust gases at approximately atmospheric pressure. With the exhaust valve remaining open, the piston now travels from BDC to TDC in the exhaust stroke. This pushes most of the remaining exhaust gases out of the cylinder into the exhaust system at about atmos- pheric pressure, leaving only that trapped in the clearance volume when the piston reaches TDC. Near the end of the exhaust stroke bTDC, the intake valve starts to
Sec. 1-6 Basic Engine Cycles 27 open, so that it is fully open by TDC when the new intake stroke starts the next cycle. Near TDC the exhaust valve starts to close and finally is fully closed sometime aTDC. This period when both the intake valve and exhaust valve are open is called valve overlap.
Four-Stroke CI Engine Cycle
1. First Stroke: Intake Stroke The same as the intake stroke in an SI engine with one major difference: no fuel is added to the incoming air.
2. Second Stroke: Compression Stroke The same as in an SI engine except that only air is compressed and compression is to higher pressures and temperature.
Late in the compression stroke fuel is injected directly into the combustion chamber, where it mixes with the very hot air. This causes the fuel to evaporate and self-ignite, causing combustion to start.
3. Combustion Combustion is fully developed by TDC and continues at about constant pressure until fuel injection is complete and the piston has started towards BDC.
4. Third Stroke: Power Stroke The power stroke continues as combustion ends and the piston travels towards BDC.
5. Exhaust Blowdown Same as with an SI engine.
6. Fourth Stroke: Exhaust Stroke Same as with an SI engine.
Two-Stroke SI Engine Cycle (Fig. 1-17)
1. Combustion With the piston at TDC combustion occurs very quickly, rais- ing the temperature and pressure to peak values, almost at constant volume.
2. First Stroke: Expansion Stroke or Power Stroke Very high pressure cre- ated by the combustion process forces the piston down in the power stroke. The expanding volume of the combustion chamber causes pressure and temperature to decrease as the piston travels towards BDC.
3. Exhaust Blowdown At about 75° bBDC, the exhaust valve opens and blowdown occurs. The exhaust valve may be a poppet valve in the cylinder head, or it may be a slot in the side of the cylinder which is uncovered as the piston approaches BDC. After blowdown the cylinder remains filled with exhaust gas at lower pressure.
4. Intake and Scavenging When blowdown is nearly complete, at about 50°
bBDC, the intake slot on the side of the cylinder is uncovered and intake air-fuel enters under pressure. Fuel is added to the air with either a carburetor or fuel injec- tion. This incoming mixture pushes much of the remaining exhaust gases out the open exhaust valve and fills the cylinder with a combustible air-fuel mixture, a process called scavenging. The piston passes BDC and very quickly covers the intake port and then the exhaust port (or the exhaust valve closes). The higher pres-
Sec. 1-6 Basic Engine Cycles 29 sure at which the air enters the cylinder is established in one of two ways. Large two- stroke cycle engines generally have a supercharger, while small engines will intake the air through the crankcase. On these engines the crankcase is designed to serve as a compressor in addition to serving its normal function.
5. Second Stroke: Compression Stroke With all valves (or ports) closed, the piston travels towards TDC and compresses the air-fuel mixture to a higher pres- sure and temperature. Near the end of the compression stroke, the spark plug is fired; by the time the piston gets to IDC, combustion occurs and the next engine cycle begins.
Two-Stroke CI Engine Cycle
The two-stroke cycle for a CI engine is similar to that of the SI engine, except for two changes. No fuel is added to the incoming air, so that compression is done on air only. Instead of a spark plug, a fuel injector is located in the cylinder. Near the end
Figure 1-18 1996 General Motors L67 3800 Series II spark ignition, four-stroke cycle, overhead valve, 3.8 liter, V6 engine. This supercharged engine has two valves per cylinder and has power and torque ratings of 240 hp (179 kW) at 5200 RPM and 280 Ibf-ft (380 N-m) at 3600 RPM. Copyright General Motors Corp., used with permission.
